Conformational studies of omega-conotoxins using electrospray mass spectrometry

MVIIA and MVIIB omega-conotoxins were chosen to investigate the effect of experimental conditions on their conformations, because of the presence of three disulfide bridges in these toxins. There were no significant effects of ion-source temperature, cone voltage, pH and percentage of cosolvent. We show that charge state distributions (CSDs) observed in their electrospray mass spectra are not a true reflection of the behaviour in the bulk solution because of electrostatic effects during the ion-evaporation process in the ion source. As a result it is not possible to deduce from the observed CSDs that some basic amino acids are hidden in the core of the peptide structure. This is important in view of the complementary finding that nearly all labile hydrogens are rapidly exchanged in deuterated solvents. The mass spectrometry results can be reconciled with results of NMR experiments and molecular calculations from the literature.